First, if it is aluminum I just flat would not use it no matter what the pressure rating.
If it is steel I think those are 3000 psi rated. However, I am 99% certain that the end plugs are aluminum, and must be carefully examined for corrosion and cracks. If these have been wet, the electrolysis between the steel and Aluminum can damage the integrity of the plugs.
Your concerns about the expansion of escaping air are certainly valid, and I have always been troubled about ththose retainer rings (no I did not accidently misspell "those"... I was stuttering at the thought of them), although these surplus accumulators were in common use in the 60's and 70's. The only accident with these exact units that I personally know of was at Lake Whitney, Texas, in the 70's. One of the ends blew out, sending a missile up through the roof. I don't know what parts hit the owner (I don't think the tube split), but he sustained a badly broken arm and a case or ringing ears. Made a mess in the shop (and maybe in his shorts). He felt lucky to be alive. If I recall he quit pumping air.
The cause could have been a failure to fully seat the retaining ring into its groove... or it could have been a failure of actual ring, or of the metal groove. I doubt that was ever determined for sure, but I believe it could easily have been user error.
Anyway, they scare me.
I think these may have been made as lightweight as possible for use in aircraft, and maybe never intended for such long term use. I know the guy that built that compressor system, and will try to find out more from him. Of course there was legal action over the event, and he may still be wanting to just cover his backside and not talk about it. I'll post it here if I can obtain any real facts from him.
New filter towers are expensive, but so are medical bills, building repairs, and of course funerals have gotten very expensive too. Just sayin'....